1. Field of the Invention
The present invention relates to an electric power conversion device with push-pull circuitry, in which a DC power supply such as a solar cell or a fuel cell is switched on/off to generate high-frequency pulses which are rectified and smoothed through a pulse transformer thereby to generate a direct current, or in particular an electric conversion device with push-pull circuitry for preventing the magnetic deviation phenomena of the pulse transformer due to an unbalanced current of the push-pull circuit.
2. Description of the Related Art
In recent years, what is called the parallel operation system with mains has been provided, in which DC power obtained from a solar cell or a fuel cell is converted into AC power as a distributed power generation unit, and power is supplied to domestic appliances (loads) using the distributed power generation unit in collaboration with the commercial power supply, so that in the case where the power demand cannot be met by the distributed power generation unit, power is supplied from the commercial power supply.
In this parallel operation system with mains, a power conditioner is used for outputting by converting the DC output power of the solar cell or the fuel cell to the AC current or power synchronous with the system to make possible parallel connection with the commercial power supply or the like AC power supply. This power conditioner is configured of a power conversion device for converting the DC power to the AC power synchronous with the commercial power supply (say, AC 200 V) and a protective device for detecting a fault of the commercial power supply.
Also, the power conversion device generates high-frequency pulses from the direct current of the solar cell or the fuel cell through a switching element such as an FET (field effect transistor), applies the high-frequency pulses thus generated to the primary side of the pulse transformer, and rectifies and smoothes the high-frequency pulses obtained from the secondary side of the pulse transformer, thereby generating the DC current again, which is converted to an AC power supply through an inverter.
The electric power conversion device also comprises a DCxe2x80x94DC converter for turning on/off two switching elements alternately thereby to convert the DC current into high-frequency pulses, which are applied to the two windings on the primary side of the pulse transformer, and rectifying and smoothing the high-frequency pulses of opposite polarities obtained from the two secondary windings of the pulse transformer thereby to generate a DC current. This DC power is converted to the AC power synchronized (say, at 50 Hz) with the commercial power supply through an inverter.
The conventional electric power conversion device of this type is known to include a DCxe2x80x94DC converter with push-pull circuitry configured of two switching elements and a pulse transformer. The electric power conversion device with push-pull circuitry has the advantage that the circuit can be reduced in size with a higher power conversion efficiency.
With the conventional power conversion device with push-pull circuitry, in the case where the switching elements making up the push-pull circuit have different on time or in the case where the values of the positive and negative current undergo a change due to a load variation, the problem of magnetic deviation phenomena is posed in which the magnetic flux density of the iron core of the pulse transformer is deviated to the winding having a longer on time or having a larger current value. Thus, the deviation of the magnetic flux density is increased for each switching period, and finally saturated by exceeding the maximum magnetic flux density of the iron core, with the result that the inductance of the winding having a longer switching on time or a larger current amount is reduced for a further increased coil current.
Several conventional power conversion devices with push-pull circuitry, which find applications for suppressing or preventing the magnetic deviation phenomena, harbor the problem that the circuit configuration and the structure of the pulse transformer are complicated. An example of the practical application is a method in which the transformer and the FET of the switching elements have a margin of capacity to prevent the damage due to an increased coil current caused by the magnetic deviation phenomena. This method, however, leads to an increased device size and an increased cost. Another practical example is a method in which a thermocouple is mounted on the transformer to detect the temperature rise due to the DC magnetic deviation. The problem of this method lies in the process of mounting by bonding or fixing the thermocouple on the transformer core. Also, the detection of a temperature rise is difficult before actual temperature rise, resulting in a low response of compensation for the DC magnetic deviation. Still another example of the conventional application is a method in which an increased primary current of the transformer due to the DC magnetic deviation is detected by a CT (current transformer). In this method, the primary current of the transformer is required to be constantly monitored by inserting the CT on the primary side of the transformer. This requires a high-speed current detection and a high-speed arithmetic operation, which in turn requires the provision of an expensive operational IC and I/F circuit.
This invention has been developed to solve the problems described above, and the object thereof is to provide a power conversion device with push-pull circuitry having a simple configuration for preventing the DC magnetic deviation of the pulse transformer.
In order to solve the aforementioned problems, according to one aspect of the invention, there is provided a power conversion device with push-pull circuitry comprising a DC magnetic deviation prevention means for detecting the coil current flowing in the two primary windings of a pulse transformer and balancing the coil current by adjusting the on time of two switching elements based on the coil current deviation, thereby preventing the DC magnetic deviation of the primary winding of the pulse transformer.
The power conversion device with push-pull circuitry according to this aspect of the invention comprises a DC magnetic deviation prevention means for detecting the coil current flowing in the two primary windings of a pulse transformer and balancing the coil current by adjusting the on time of two switching elements based on the coil current deviation, thereby preventing the DC magnetic deviation of the primary winding of the pulse transformer. Therefore, a switching element can be specified by the coil current deviation and the polarity (+ or xe2x88x92) of the coil current deviation, and the same coil current flowing in the two primary windings of the pulse transformer can be secured by determining the on time, thereby preventing the DC magnetic deviation of the pulse transformer with a simple configuration.
According to another aspect of the invention, there is provided a power conversion device with push-pull circuitry, comprising a magnetic prevention means including a current sensor for detecting the coil currents flowing in the two primary windings of a pulse transformer, an integration means for integrating the detected coil current for a predetermined period of time, a drive control means for outputting a drive control signal corresponding to the integration value accumulated by the integration means, and a drive means for outputting a drive signal for adjusting the on time of the two switching elements based on the drive control signal supplied from the drive control means.
The DC magnetic-deviation prevention means according to this aspect of the invention includes a current sensor for detecting the coil currents flowing in the two primary windings of a pulse transformer, an integration means for accumulating the detected coil current for a predetermined period of time, a drive control means for outputting a drive control signal corresponding to the integration value accumulated by the integration means, and a drive means for outputting a drive signal for adjusting the on time of the two switching elements based on the drive control signal supplied from the drive control means. Therefore, a switching element for supplying a greater amount of coil current is specified based on the detected coil current, and the coil currents flowing in the two primary windings of the pulse transformer can be balanced by adjusting the on time of the specified switching element, thereby making it possible to prevent the magnetic deviation phenomena of the pulse transformer which otherwise might be caused by the unbalanced coil currents.
According to still another aspect of the invention, there is provided an electric power conversion device in which the current sensor is configured of a CT (current transformer) and the coil currents flowing in the two primary windings of the pulse transformer are detected in reverse polarities.
With the current sensor making up a CT according to this invention, the coil currents flowing in the two windings on the primary side of the pulse transformer are detected in reverse polarities, and therefore the deviation of the coil currents flowing in the two windings can be detected with a single CT, thereby simplifying the device.
According to yet another aspect of the invention, there is provided an electric power conversion device, in which the drive control means determines a switching element for controlling the on time based on the polarity of the integration value accumulated by the integration means.
With the drive control means according to this aspect of the invention, the switching element for controlling the on time is determined based on the polarity of the integration value accumulated by the integration time, and therefore the on time of the switching element supplied with a greater amount of the coil current can be controlled, thereby making it possible to balance the coil currents flowing in the two primary windings of the pulse transformer.
According to a further aspect of the invention, there is provided an electric power conversion device, in which the DC power supply is a solar cell or a fuel cell.
In this aspect of the invention, the DC power supply is a solar cell or a fuel cell. Therefore, the DC power supply used as a distributed power generation unit for the domestic electric appliances can find wider applications, thereby making it possible to improve the utility of the electric power conversion device.